Continuous process for obtaining pure alumina from an acidic liquor originating from the chlorosulphuric attack of an aluminous ore and for the purification of the liquor which has been freed from alumina

Abstract

A process for obtaining pure alumina comprising the combination of chlorosulphuric attack of calcined aluminous ores containing impurities, followed by concentration of the liquor obtained after the attack and separation of the sterile material, sulphuric acid treatment of the concentrated liquor followed by chlorination to precipitate almost all the hexahydrated aluminum chloride, separation of the precipitate of hexahydrated aluminum chloride and of the chlorosulphuric mother liquor charged with impurities, pyrohydrolysis of the precipitate to give the desired pure alumina with recycling of released hydrochloric acid, degassing of the chlorosulphuric liquor of impurities with recycling of the gaseous HCl for chlorination, yielding a sulphuric liquor of impurities, finally the elimination of the impurities present in this latter liquor in the form of a sulphuric precipitation by concentration in the presence of potassium ions.

We claim: 
     
       1. A process for obtaining a purified alumina from calcined aluminous ore containing impurities of metals selected from the group consisting of iron, titanium, sodium, potassium, magnesium and calcium comprising: (a) attacking the ore with hot chlorosulphuric acid to provide a residue and a liquor containing dissolved alumina and impurities,   (b) separating the residue from the liquor,   (c) washing the separated residue with aqueous medium to extract the solubles,   (d) recycling the wash liquor from step (c) to make up a part of the chlorosulphuric acid attacking liquor in step (a),   (e) concentrating the liquor separated in step (b) to the limit of solubility of aluminum chloride hexahydrate,   (f) adding sulphuric acid liquor to the concentrate of step (e),   (g) introducing HCl gas recycled from step (j) into the mixture from step (f) to precipitate almost all of the aluminum chloride hexahydrate,   (h) separating the liquor containing the impurities from the precipitate from step (g),   (i) degassing the liquor separated in step (h) to release HCl gas,   (j) recycling the gaseous HCl for introduction into the mixture in step (g) to precipitate almost all of the aluminum chloride hexahydrate,   (k) adding potassium ions to the degassed liquor from step (i),   (l) concentrating the liquor from step (k) to precipitate ferripotassium sulphates and sulphates of other impurities,   (m) separating the sulphuric liquor from the precipitate formed in step (l),   (n) recycling the sulphuric liquor from step (m) to the sulphating step (f), and   (o) thermally decomposing the separated aluminum chloride hexahydrate from step (h) to produce pure alumina.   
     
     
       2. A process as claimed in claim 1, characterized in that the attacking liquor contains about 20% of HCl and 5% of H 2  SO 4 . 
     
     
       3. A process as claimed in claim 1, in which the attack is performed at a temperature close to the boiling point. 
     
     
       4. A process as claimed in claim 1, which includes the step of mixing the concentrated chlorosulphuric mother liquor originating from the attack, with a recycled sulphuric liquor containing from 40 to 65% of free H 2  SO 4 . 
     
     
       5. A process as claimed in claim 1, which includes the step of calcining the ferripotassium sulphates and sulphates of impurities, transforming the SO 2  obtained into H 2  SO 4  for recycle into the treatment circuit. 
     
     
       6. The process as claimed in claim 1 which includes the step of: (p) washing the separated precipitate from step (h) with hydrochloric acid liquor recycled from step (s)   (q) recycling the liquor separated from the precipitate of step (p) to form a part of the chlorosulphuric attack liquor in step (a),   (r) calcining the precipitate from step (p) to yield purified aluminum oxide and HCl gas, and   (s) absorbing the HCl gas from step (q) to form hydrochloric acid recycled to the wash step (p).   
     
     
       7. The process as claimed in claim 6 in which HCl gas is liberated during the concentration of the liquor in step (l) and which includes the step of absorbing the HCl gas to form a hydrochloric acid liquor recycled to wash the precipitate in step (p).